Whats wrong with my ford focus 55 plate 1.8TDCi?

My Focus seems to be driving fine and then suddenly an Engine Systems Fault shows up on the dash, the car loses power and eventually stops and will not re-start. Acouple of garages have looked at it and got it re-started but then can't find anything wrong. IT seems to break down during or just after we've had lots of wet weather I don't know if that is a coincidence. Please help!